The production and separation of paraxylene are carried out in industrial practice by arranging the following in a loop:
a process for separation of the paraxylene by adsorption (U.S. Pat. Nos. 5,284,992; 5,401,476; and 5,329,060), whose effluents are paraxylene, on the one hand, and an aromatic C.sub.8 petroleum fraction that is substantially free of paraxylene, on the other. Crystallization can be combined with the adsorption stage to obtain purer paraxylene (U.S. Pat. No. 5,284,992, U.S. Pat. No. 5,401,476, U.S. Pat. No. 5,329,060); PA1 a process for isomerizing the aromatic C.sub.8 petroleum fraction, whereby said process treats the second of the two effluents of the separation unit and produces an isomerate that contains paraxylene. This isomerate is recycled to the feedstock stream that feeds the paraxylene separation unit.
There are two classes of processes for isomerization of paraxylene: the first class is known by the term "converting isomerization" because ethylbenzene is in part converted into xylenes, which are in proportions that are close to those of the thermodynamic equilibrium. The catalysts that are used in the conversion isomerization steps are bifunctional. A zeolite, such as, for example, mordenite, ensures the conversion of orthoxylenes and metaxylenes into paraxylene by migration of the methyl groups. As a result, at the temperature in question, thermodynamic equilibrium is nearly reached among the three xylenes: at 400.degree. C., typically, orthoxylene 24%, metaxylene 52%, and paraxylene 24%. Dispersed platinum ensures, in the presence of hydrogen, a hydrogenating-dehydrogenating function that makes it possible to convert the ethylbenzene in to a mixture of xylenes. Hydrogen is necessary to produce the naphthenic intermediate chemicals that yield xylenes after dehydrogenation.
The operating conditions of the isomerization are dictated by the conversion of ethylbenzene: temperature and partial pressure of hydrogen. The commercially available catalysts only ensure on the order of 40% of conversion per pass and require that a significant proportion of naphthenes be present in the loop. The applied temperature is increased to ensure the desired paraxylene production. Taking into account the compositions of the fresh feedstock and of the isomerate, it is necessary to treat the flow of fresh feedstock 3 to 5 times in the separation unit to produce about 0.85 times the flow of fresh feedstock in the form of paraxylene. The 10 to 20% of fresh feedstock that is not converted into paraxylene is found in the form of cracking and transalkylation products.
The second class of isomerization processes is known by the name dealkylating isomerization.
In this type of isomerization, ethylbenzene is converted into benzene and ethylene on catalysts with a zeolite ZSM5 base, while the xylenes are brought into thermodynamic equilibrium. Hydrogen is also needed here to hydrogenate into ethane the ethylene that is formed (to prevent realkylation) and to prevent the coking of the catalyst. The H.sub.2 /HC ratio, however, is considerably lower than that found in converting isomerization. In this case, co-production in the separation-isomerization loop of paraxylene (about 78%) and benzene (15%), with 7% of various losses, is ensured. Here again, the temperature conditions are still dictated by the fact that it is necessary to dealkylate the ethylbenzene.
In contrast, in industrial practice, ethylbenzene is the reaction intermediate chemical that makes it possible to obtain styrene by dehydrogenation. Ethylbenzene is always produced by alkylation of benzene with ethylbenzene. These alkylation units require a reactor with considerable recycling to be able to control the exothermicity of the reaction and, moreover, a number of distillations finally to separate gases, benzene, ethylbenzene, and di-, tri- and tetraethylbenzene.
Molecular sieves that can separate ethylbenzene from xylenes have been described effectively (U.S. Pat. No. 4,497,972, U.S. Pat. No. 5,433,560). Despite the respectable separation performance of these sieves, to our knowledge no commercial unit for separation of ethylbenzene in a simulated fluid bed has been built to date.
The prior art actually has always regarded the production of ethylbenzene as an isolated problem. If it is considered that the aromatic C8 feedstocks from which ethylbenzene is to be extracted contain at most 16%, a process for separating ethylbenzene, such as, for example EBEX.RTM., is more expensive than a unit for alkylating benzene. This way of looking at things has quite often been reinforced by the fact that the locations where paraxylene and orthoxylene, on the one hand, and styrene, on the other, are produced are generally geographically separate: actually, the xylene production line is most often integrated into a refinery to keep from having to transport the aromatic C.sub.8 petroleum fraction. In some cases, however, it is integrated into a plant for producing terephthalic acid or methyl terephthalate. By contrast, the ethylbenzene production line in generally integrated into a plant for producing styrene and polystyrene.